Self-disclosure is when you tell someone something about yourself. It's like when you tell your best friend what your favorite toy is or what you like to do for fun. It's when you share a part of yourself with someone else.
Adults do self-disclosure too, but it's usually a bit more complicated. Grown-ups might tell someone about their job, their family, or their feelings. When they share these things, it can help them feel closer to the person they're talking to.
But even though self-disclosure can make people feel closer, it's important to think before you say too much. You wouldn't want to tell someone something private that you didn't want others to know. So, just like when you decide to share with your best friend, it's important for grown-ups to think about what they want to say and who they want to say it to.